Synchronous rectifiers improve the efficiency of all DC-DC converters by reducing the power dissipated in the output rectification stage. Power dissipation is decreased by reducing the output voltage drop in the rectification stage. By replacing diodes with devices having lower voltage drops, efficiency will be dramatically increased, particularly at low output voltage levels in the range of 1.5 to 3.3 volts. This is the Synchronous Rectification concept, using MOSFETS instead of diodes as rectifiers.
Control of synchronous rectifiers in DC-DC converters conventionally have been accomplished by self-driven and control driven circuitry. Each method does not prevent current spikes, or cross-conduction, as the current switches from one MOSFET to the other. In self-driven circuitry, the diodes are replaced by MOSFET switches having their gates connected to the secondary winding of the isolation transformer. Although this method is simple and has low MOSFET driving losses, the driving voltage and timing are highly dependent on the converter topology and limits the useable input voltage range since the gate drive voltage varies with input voltage.
Control-driven techniques, though more complex than self-driven methods, can overcome the limitations thereof except for preventing cross conduction. Control-driven techniques offer constant gate drive voltage but have driving losses and additional cost because of added parts. This method allows use of several power topologies without input voltage limitations.
Accordingly, it is desirable to provide an apparatus for preventing cross conduction in a synchronous rectifier. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.